The Junyos were laid down in 1939 as
27,500-ton passenger linersKashiwara Maru and Izumo Maru. Part of the "shadow
program," they were designed with destroyer
machinery and were converted to
light carriers on the ways.
They retained much of their civilian character, including clean hulls,
forward sheer, and inadequate subdivision. Though rated
as fleet carriers on account of
their relatively
large air group, their speed was inadequate for fleet operations. The
two hangar decks were
cramped, the lower having barely enough ceiling clearance for the A6M "Zero", let alone carrier bombers. The ships had
almost no armor
protection, which with their poor subdivision meant that their
survivability in battle was minimal. However, they were the first
Japanese carriers to direct their uptakes through the island, and the
island was sponsoned out so that it did not encroach on the flight
deck. The hulls were asymmetrically bulged to compensate for the island.

Junyo had
four boilers rather than the six of Hiyo,
giving her a slightly slower speed of 24 knots.

The original air group was 18 D3A "Val", 18 B5N "Kate" and 12 A6M "Zero" plus a
few spares. Like the Americans,
the Japanese seem to have increased the proportion of fighters in their air groups as the
war progressed, and Hiyo was
operating 27 "Zeros", 6 B6N "Jill",
and 18 "Vals" by 1944.